1. Field of the Invention
This invention relates to an optical system for determining the spatial dimensions of an object. More particularly, a sensor is angled relative to an image plane of the optical system. Various areas of the sensor in the image plane focus to a slightly different elevation. Electronic processing is employed to determine which linear scan is most in best focus. From the best focus data, both the height of the object relative to a base surface and the coplanarity of the object relative to other objects may be rapidly determined.
2. Description of the Related Art
Semiconductor packages intended to encapsulate one or more semiconductor microelectronic devices include a feature to transmit electrical signals from external sources to the devices. While leadframes are widely used to transmit the electrical signals, the leadframe extends beyond the semiconductor package periphery extending the area on a printed circuit board required to support that device. As space is often a consideration, leadless semiconductor packages have been developed. One such leadless semiconductor package is referred to as a ball grid array (BGA) package.
In a BGA package, solder balls are mounted to bond pads formed on either the semiconductor base or cover. Conductive vias extend from the bond pads to electrically conductive sites in the semiconductor package interior. The electrically conductive sites are electrically interconnected to the one or more microelectronic devices such as through small diameter bond wires. The resulting structure provides electrical continuity from the microelectronic device to the solder balls.
When the semiconductor package is to be joined to a printed circuit board, the package and board are heated to a temperature sufficient to melt the solder balls that then fuse with bond pads formed on a surface of the printed circuit board. The solder balls are relatively small, on the order of 0.040 inch in diameter, and closely spaced one from another with a typical solder ball to solder ball pitch being 0.080 inch.
During the fusing step, adjacent solder balls must not contact one another or a short circuit will form. In addition, each solder ball must contact a matching bond pad on the printed circuit board or an open circuit will form. To prevent these defects, both the solder ball size (solder volume) and solder ball coplanarity (solder ball diameter) must be tightly controlled.
There are a number of optical systems presently available to measure solder balls in a BGA package as a quality control function prior to sing. These systems typically require a significant amount of time to evaluate each solder ball, on the order of up to 1 second per BGA package. Manufacturers seek a method to characterize the solder balls while reducing the inspection time.
Among the solder ball inspection systems presently utilized are systems that determine BGA characteristics based on comparing the views of two separate video cameras. Just as human eyes perceive depth based on the slight physical offset between the eyes, these systems utilize the same parallax concept to determine the height of each bump by comparing the differences between the two views of each camera. The manufacturers claim accuracy of 10 to 12 microns for coplanarity and diameter of each ball with an inspection throughput of 0.250 to 1 second per part. The part must come to a stop in front of the inspection cameras during use.
Another inspection system utilizes Fast Moirxc3xa9 Interferometry. A dense pattern of light and dark areas are projected onto the solder balls while a video camera images the subject. The pattern is physically shifted slightly and the video camera acquires a second image of the part. This may be done once or multiple times and the various views are recorded by the system. If the pattern is fairly ordered, say a series of rectangles, as they fall across the spherical shapes of the solder balls, the system can determine the position and form of the balls. Shifting the projected pattern a given amount and reviving the changes in the image enable the system to determine ball height and ultimately coplanarity. This system is claimed by the manufacturer to be slightly faster and more accurate than systems based on the parallax concept.
Still another measuring system determines the three dimensional form of objects, such as solder bumps, by projecting a thin line of laser light on a part. A video camera takes an image of the line of laser light. For a flat object, the image of the laser line is simply a line. If solder bumps are introduced, the laser line will appear to have curves proportional to the size of the bumps. The speed of this system is limited by the acquisition speed of the video camera.
Other methods for bump height measurement currently employed are microscope auto-focus and laser point triangulation. These methods are significantly slower than those discussed above and are not believed used for in-line 100% package inspection.
U.S. Pat. No. 6,134,013, to Sirat et al., discloses a method and apparatus for inspecting a three dimensional structure such as an electronic component, using collimated light for accurately measuring the three-dimensional structure of members protruding above the surface of the component. Collimated light illuminates the surface and a spherical or other three-dimensional structure on the surface is analyzed using stereoscopic, holographic, moirxc3xa9, or conoscopic analysis of light reflected or scattered by the structure, using an array of optical detectors in a detector plane.
U.S. Pat. No. 6,144,453, to Hallerman et al., discloses a three-dimensional inspection system used to obtain three-dimensional information pertaining to an article having specular surfaces, such as a BGA device, by projecting a pattern of light onto the article at an oblique angle with respect to the article. The system includes a patterned light projector having an optical axis disposed at an oblique angle with respect to the plane of the article being inspected and an image detector or camera disposed generally above the article being inspected to detect the image of the pattern projected on the article. The pattered light projector includes an extended light source that directs light along the optical axis and a light patterning member disposed at an angle with respect to the optical axis such that the light pattern is in focus in an image plane parallel to the plane of the article, thereby satisfying the Scheimpflug condition (to avoid distortion, film plane, subject plane and lens plane converge along a single line). The light pattern preferably includes lines of light projected onto the article with substantially equal thickness and spacing. The spacing of the lines is preferably greater than a spacing or pitch of the specular elements. An image processor is coupled to the image detector to receive the image, locate the lines, and measure the lateral shift of the lines. Height information is determined from the lateral shift and projection angle using triangulation.
Both the U.S. Pat. No. 6,134,013 and the U.S. Pat. No. 6,144,453 is incorporated by reference herein in their entireties.
U.S. Pat. No. 6,160,910 to Freifeld discloses a high precision camera effective to capture a video image of an object with enough precision such that information derived form the video image is sufficient to provide accurate measurements of the object for quality control and other applications. A positional encoding technique that is highly insensitive to transient and steady state mechanical tolerances in the mechanical scanning system is employed. The U.S. Pat. No. 6,160,910 is incorporated by reference herein in its entirely.
While the above-noted methods have served the industry well, there remains room for improvement. The parallax and moire systems require the part to stop in front of the camera for at least 0.2 seconds. Ideally the industry would like a sensor where the part would not have to stop at all. In addition, these methods do not provide a full 360-degree height representation of the ball but instead characterize the surface at the tops of the ball and a portion of one side.
Accordingly, it is an object of the invention to provide an inspection system to characterize the height and surface form of a three dimensional object. Another object of the invention is that the inspection is essentially dynamic, that is he pard is not stopped in front of a camera during inspection, or if stopped, the stop time is minimal.
It is a feature of the invention that, in one embodiment, the inspection system utilizes a plurality of detectors positioned at different heights in the optical axis near the image plane of a lens. A computer system receives data from each detector and determines the distance between the object and the lens based on a contrast comparison of a pixel and its neighbors at various heights; it is recognized that where the contrast is at a maximum the object is typically in best focus.
It is an advantage of the system of the invention that three-dimensional objects, such as solder bumps projecting from a semiconductor package, are measured more accurately, more completely and faster than by current techniques. A further advantage of this invention is that the inspection system does not require a pat to stop in front of the inspection camera, but merely to pass by it while in motion.
In accordance with a first embodiment of the invention, there is provided a system for measuring the spatial dimensions of a three-dimensional object that employs a lens and sensor. This sensor is made up of at least two detectors. The combination of the lens and sensor defines an optical axis that is normal to the object planes and passes through a focal point of the lens. At least a first and a second photodetector or an assay of detectors are optically aligned with the object through the lens. The first and second photodetectors or array of detectors are further parallel with the object planes but have differing elevations relative to the object planes. In addition, the first and second photodetectors are capable of motion relative to the object. Electrical outputs from the first and from the second photodetectors are provided to a processor that determines a distance of the object from the lens and thereby a height of the object. It is a fiber advantage of the invention that X, Y, and Z coordinate information and gray scale information is transmitted to the processor at the same time.
In accordance with a second embodiment of the invention, a pattern of light and dark features is projected on an object This projection causes the objects to appear grainy which is an aid to focusing algorithms. A further enhancement can be achieved through the projection of a grid of light and dark areas using a back illuminated reticle.
In accordance with a third embodiment of the invention, chromatic aberration is introduced into the projecting lens.
The above stated objects, features and advantages will become more apparent from the specification and drawings that follow.